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Stock Option Plan - Narrative (Details) - USD ($)
1 Months Ended 3 Months Ended 9 Months Ended
Aug. 31, 2018
Mar. 31, 2017
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Forgiveness of notes receivable         $ 0 $ 437,000  
Early exercise of stock options, share subject to repurchase (in shares)     78,757   78,757   16,117
Liability for early exercise of unvested stock options     $ 359,000   $ 359,000   $ 65,000
Stock options granted (in shares)         73,783    
Weighted-average exercise price of stock options granted (in dollars per share)     $ 9.88   $ 5.51    
Unrecognized compensation cost     $ 1,900,000   $ 1,900,000    
Minimum              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Exercise price of option as percent of fair market value when individual owns more than 10% of shares outstanding (percent)         110.00%    
Incentive Stock Options | Minimum              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Exercise price of option as percent of fair market value (percent)         100.00%    
Nonqualified Stock Options | Minimum              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Exercise price of option as percent of fair market value (percent)         85.00%    
Stock Options              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Expiration period (in years)         10 years    
Expiration period when individual holds more than 10% of shares outstanding (in years)         5 years    
Award vesting period (in years)         4 years    
Stock options granted (in shares)     19,998        
Unrecognized compensation cost, expected period for recognition (in years)         2 years 1 month 6 days    
Stock-based compensation expense     $ 328,000 $ 343,000 $ 1,081,000 $ 1,038,000  
Performance Shares              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Awards granted in period (in shares)   564,098          
Weighted-average grant date fair value (in dollars per share)   $ 5.94          
Stock-based compensation expense         $ 0    
Performance Shares | Vesting Tranche One              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Award vesting period (in years)   4 years          
Awards granted in period (in shares)   408,544          
Performance Shares | Vesting Tranche Two              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Award vesting period (in years)   3 years          
Awards granted in period (in shares)   155,554          
Restricted Stock Units              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Awards granted in period (in shares) 37,036            
Weighted-average grant date fair value (in dollars per share) $ 7.74            
Restricted Stock Units | Vesting Tranche One              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Award vesting period (in years) 1 year            
Awards vesting rights (percent) 50.00%            
Restricted Stock Units | Vesting Tranche Two              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Award vesting period (in years) 2 years            
Awards vesting rights (percent) 50.00%            
2008 Stock Option Plan              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Number of shares authorized (in shares)             5,350,080